click on the Reorient button and press done (your image is already selected). If you want to apply the same transformation to other images (e.g. if you have a series of functional images), select them all at this stage.

say No to Do you want to save the reorientation matrix?

This will set the origin of the image (0 0 0 mm coordinates) to AC. You might also want to rotate the image such that it is better aligned with MNI space: to do so, you also need to edit the entries for the rotations (in radian) along the pitch, roll and yaw axes.

Use the ImCalc facility with expression "mean(X)" and Data Matrix option set to "yes - read images into data matrix". You can also choose the output filename (say mean.img) for the mean image and its directory (default is current folder).

Alternatively, you can use the spm_mean_ui function but the number of options is limited - ImCalc should be preferred.

If you change the location of your data and/or the SPM installation, the full paths contained in batch files and SPM.mat will be broken. An utility function spm_changepath is available in SPM to replace all occurrences of one string with another within a MAT-file. For example:

spm_changepath('SPM.mat','C:\mydata','D:\Experiments\data');

will update the SPM.mat file to replace all occurrence of C:\mydata with D:\Experiments\data. A backup of the initial file can be found in SPM.mat.old. A list of all the altered paths will be displayed at runtime.